Coding an audio signal

ABSTRACT

In the method of coding the audio signal, the values of first parameters (P 1,1 ), which represent aspects of the audio signal at a first instant (ti), are calculated to obtain first calculated values (A 1, i). The values of second parameters P 2, i), which represent the aspects of the audio signal at a second, later, instant (t 2 ), are calculated to obtain the second calculated values (A 2, i). The number of the first parameters (P 1 ,i) and the number of the second parameters (P 2 ,i) differ. A subset (SUS 2 ,i) of the second parameters (P 2 ,i) is associated with a particular portion (SFRAi) of a frequency range (FR) of the audio signal This frequency range (FR) of the audio signal is preferably selected to cover all the frequencies present in the audio signal. The values (A 2, i) of the subset (SUS 2, i) of the second parameters (P 2, i) are coded based on a difference of this subset (SUS 2, i) and a subset (SUS 1, i) of the first calculated value(s) (A 1 ,i) associated with substantially this same particular portion (SFRAi) of the frequency range (FR). Thus the differentially coded values ( 7 ) of the second parameters (P 2, i) are obtained by coding the difference of the values of second parameters (P 2, i and first parameters (P 1 ,i) which are associated with substantially the same frequency subrange (SFRAi). This allows to differential code the parameters (P 1 ,I P 2 ,i) even if the number of the parameters changes in time.

The invention relates to a method of coding an audio signal, an encoderfor coding an audio signal, and an apparatus for supplying an audiosignal.

Prior solutions in audio coders that have been suggested to reduce thebit rate of stereo program material include intensity stereo and M/Sstereo.

In the intensity stereo algorithm, high frequencies (typically above 5kHz) are represented by a single audio signal (i.e., mono) combined withtime-varying and frequency-dependent scale factors or intensity factorswhich allow to recover a decoded audio signal which resembles theoriginal stereo signal for these frequency regions.

In the M/S algorithm, the signal is decomposed into a sum (or mid, orcommon) signal and a difference (or side, or uncommon) signal. Thisdecomposition is sometimes combined with principle component analysis ortime-varying scale factors. These signals are then coded independently,either by a transform-coder or sub-band-coder (which are bothwaveform-coders). The amount of information reduction achieved by thisalgorithm strongly depends on the spatial properties of the sourcesignal. For example, if the source signal is monaural, the differencesignal is zero and can be discarded. However, if the correlation of theleft and right audio signals is low (which is often the case for thehigher frequency regions), this scheme offers only little bit ratereduction. For the lower frequency regions M/S coding generally providessignificant merit.

Parametric descriptions of audio signals have gained interest during thelast years, especially in the field of audio coding. It has been shownthat transmitting (quantized) parameters that describe audio signalsrequires only little transmission capacity to re-synthesize aperceptually substantially equal signal at the receiving end. One typeof parametric audio coders focuses on coding monaural signals, andstereo signals are processed as dual mono signals.

Another type of parametric audio coders is disclosed in EP-A-1107232.This parametric audio encoder uses a parametric coding scheme togenerate a representation of a stereo audio signal which is composed ofa left channel signal and a right channel signal. To efficiently utilizetransmission bandwidth, such a representation contains informationconcerning only a monaural signal which is a combination of the leftchannel signal and the right channel signal, and parametric information.The stereo signal can be recovered based on the monaural signal togetherwith the parametric information. The parametric information compriseslocalization cues of the stereo audio signal, including intensity andphase characteristics of the left and the right channel.

The parametric information is represented by parameters whichcharacterize aspects of the audio signal in a frequency range of theaudio signal for which the parameter is determined. The coded audiosignal may comprise the coded monaural audio signal and a single globalparameter (or a set of global parameters) which are determined for thecomplete bandwidth or frequency range of the audio signal to be coded,and/or one or more local parameters (or sets of local parameters) whichare determined for corresponding sub-ranges of the frequency range ofthe audio signal (these sub-ranges of the frequency range are alsoreferred to as bins).

Many audio coding schemes employ parameters of which the amount variesover time, for example, in waveform-coders like MPEG-1 Layer-III (mp3),AAC (Advanced Audio Coding), the number of MDCT (modified discretecosine transfer) coefficients can vary over time.

The not yet published European patent application no. 2002 02076588.9(attorney's docket PHNL020356) discloses that the number of frequencysub-ranges (also referred to as bins) used for the parametric stereorepresentation can change from frame to frame.

The not yet published European patent application no. 2002 0277869.2(attorney's docket PHNL020692) discloses that the correspondingparameters of successive frames can be encoded differentially over time.In this manner, the redundancy in the time direction can be removed. Thenumber of parameters is identical in successive frames.

In E. G. P Schuijers, et.al, “Advances in Parametric coding forhigh-quality audio”, presented at 1st IEEE Benelux Workshop on Modelbased Processing and Coding of Audio (MPCA 2002), Leuven Belgium, Nov.15, 2002, a parametric coding scheme is described that has been extendedwith a parametric stereo description. This description tries to modelthe binaural cues by means of three parameters: Inter-channel IntensityDifferences (IID), Inter-channel Time Differences (ITD) andInter-channel Cross Correlation (ICC). These parameters are estimated ona non-uniform frequency grid resembling the human auditory system. Thenumber of frequency bins on this grid is typically 20. In the Europeanpatent application no. 2002 02077869.2 a scalable approach for thecoding of these parameters has been proposed.

For this parametric coding scheme also the possibility exists to changethe number of the LPC (linear Predictive Coding) coefficients used todescribe the spectral envelope from frame to frame.

A first aspect of the invention provides a method of coding an audiosignal as claimed in claim 1. A second aspect of the invention providesan encoder for coding an audio signal as claimed in claim 10. A thirdaspect of the invention provides an apparatus for supplying an audiosignal as claimed in claim 11. Advantageous embodiments are defined inthe dependent claims.

In the method in accordance with the first aspect of the invention,differential coding is performed when the number of parameters isdifferent in successive frames. This provides a more efficient coding ofthe parameters and thus less bandwidth will be required for the codedparameters.

In the method of coding the audio signal, the values of the firstparameters, which represent aspects of the audio signal at a firstinstant, are calculated to obtain the first calculated values. Thevalues of second parameters, which represent the aspects of the audiosignal at a second, later, instant, are calculated to obtain the secondcalculated values. The number of the first parameters and the number ofthe second parameters differ. A subset of the second parameters isassociated with a particular portion of a frequency range of the audiosignal. The values of the subset of the second parameters are codedbased on a difference of this subset and a subset of the firstcalculated value(s) associated with substantially this same particularportion of the frequency range.

This allows to differential code the parameters even if the number ofparameters changes over time.

In an embodiment as defined in claim 2, within a particular frequencysub-range or bin, a single parameter has to be calculated for use in thefirst frame at the first instant. Within substantially this samefrequency sub-range, several parameters have to be calculated for use inthe second frame at the second instant. Each one of the severalparameters for use in the second frame is differentially coded based onits difference with respect to the value of the single parameter.

If the frequency sub-ranges are not identical in that one of the severalparameters is associated with a frequency sub-range which is notcompletely covered by the particular frequency sub-range, a correctionmay be applied in that this parameter is coded with respect to both thesingle parameter and a parameter associated with the frequency range notcovered by the single parameter.

In an embodiment as defined in claim 3, within a particular frequencysub-range or bin, several parameters have to be calculated for use inthe first frame at the first instant. Within substantially this samefrequency sub-range a single parameter has to be calculated for use inthe second frame at the second instant. The value of the singleparameter is differentially coded with respect to the mean value of theseveral parameters.

In an embodiment as defined in claim 4, the mean value is calculated asa weighted sum of the values of the several parameters.

In an embodiment as defined in claim 5, all the weights are equal to onedivided by the number of the several parameters of the first frame whichcorrespond with the single parameter of the second frame.

In an embodiment as defined in claim 6, the weights are selected foreach one of the several parameters to correspond to the size of thecorresponding frequency sub-range.

In an embodiment as defined in claim 7, the frequency sub-ranges are notidentical in that the frequency sub-range of the single parameter onlypartly covers the frequency range of one of the several parameters, thecontribution to the mean value of the value of this one parameter isless than the other ones of the several parameters. Preferably, itscontribution depends on the percentage of the frequency range of theseveral parameters covered by the frequency sub-range of the singleparameter only partly covering the frequency range of the severalparameters.

In an embodiment as defined in claim 8, the audio signal is coded bydifferent sets of parameters. Global parameters are calculated for thetotal frequency range of the audio signal. These global parameters allowdecoding the audio signal with a basic (lower) quality. To allow animproved quality of the decoded audio signal, supplemental parametersmay be coded. The number of these supplemental parameters may changeover time. The number of the first parameters which are required duringa first frame is smaller than the number of second parameters requiredduring a successive second frame. Each one of the first parameters andthe corresponding one of the second parameters cover substantially thesame frequency sub-range. In frequency sub-ranges wherein a secondparameter value has to be coded, this parameter value is differentiallycoded with respect to the value of the corresponding first parameterwhich is associated with substantially the same frequency sub-range. Infrequency ranges for which a second parameter has to be coded but nocorresponding first parameter value is available, the value of thesecond parameter is coded differentially with respect to the globalvalue(s).

In an embodiment as defined in claim 9, the audio signal is coded bydifferent sets of parameters. Global parameters are calculated for thetotal frequency range of the audio signal. These global parameters allowdecoding the audio signal with a basic (lower) quality. To allow animproved quality of the decoded audio signal, supplemental parametersmay be coded. The amount of these supplemental parameters may changeover time. The number of the first parameters which is required during afirst frame is larger than the number of second parameters requiredduring a successive second frame. Each one of the first parameters andthe corresponding one of the second parameters cover substantially thesame frequency sub-range. In frequency sub-ranges wherein a secondparameter value has to be coded, this parameter value is differentiallycoded with respect to the value of the corresponding first parameterwhich is associated with substantially the same frequency sub-range. Infrequency ranges for which a first parameter value is available but nocorresponding second parameter has to be coded, nothing has to happen.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows a block diagram of an encoder in accordance with anembodiment of the invention,

FIG. 2 shows a schematic representation of a situation wherein thenumber of parameters during a first frame is less than during a secondframe,

FIG. 3 shows another schematic representation of a situation wherein thenumber of parameters during a first frame is less than during a secondframe,

FIG. 4 shows a schematic representation of a situation wherein thenumber of parameters during a first frame is higher than during a secondframe,

FIG. 5 shows another schematic representation of a situation wherein thenumber of parameters during a first frame is higher than during a secondframe,

FIG. 6 shows a schematic representation of a situation wherein thenumber of parameters during a first frame is less than during a secondframe, and

FIG. 7 shows a schematic representation of a situation wherein thenumber of parameters during a first frame is higher than during a secondframe.

The same references in different Figs. refer to the same signals or tothe same elements performing the same function.

FIG. 1 shows a block diagram of an encoder in accordance with anembodiment of the invention. An input IN receives an audio signal 1. Theaudio signal 1 has to be coded in such a way that a data-reduction isachieved. Data reduction is possible by representing certain aspects ofthe audio signal by parameters. These parameters define a certain aspectof the audio signal 1 within a particular frequency range of the audiosignal 1. The particular frequency range of the audio signal 1 may coverall frequencies present in the audio signal 1, or may be a sub-range ofthe frequencies present in the audio signal 1. The parameters have to bedetermined regularly in time to be able to represent the changing audiosignal 1. Usually, the parameters are determined and coded at regulartime intervals called frames. The exact way the audio signal 1 isrepresented by the parameters, and the parameters are coded is notimportant to the invention, many known approaches may be implemented.The invention is directed to the fact that the parameters aredifferentially coded, even when the number of parameters to be codeddiffers over successive frames.

A calculating unit 2 receives the audio signal 1 and supplies calculatedvalues 3 every frame. The calculated values 3 represent parameters whichshould be differentially coded. The coded values should be available ina particular frame. A memory 4 stores the calculated values 3 everyframe and supplies the stored values 5. The encoder 6 codes thedifference of the calculated values 3 of a present frame and the storedvalues 5 of the preceding frame and supplies the differentially codedparameter values 7. The differentially coded parameter values 7 may becombined with a coded monaural audio signal in the unit 8 to supply acoded audio signal 9 at the output OUT.

The encoder may contain dedicated hardware or may be a suitablyprogrammed processor which performs the calculations and the othersteps.

FIG. 2 shows a schematic representation of a situation wherein thenumber of parameters during a first frame t1 is less than during asecond frame t2. The parameters P1,1 to P1,4 (further referred to asP1,i) and their associated frequency sub-ranges SFRA1 to SFRA4 (furtherreferred to as SFRAi) are shown at the left side for a first frame t1.The parameters P2,1 to P2,16 (further referred to as P2,i) and theirassociated frequency sub-ranges SFRB1 to SFRB16 (further referred to asSFRBi) are shown the at the right side for a second frame t2 succeedingthe first frame t1.

The parameter P1,i has a calculated value Ai, and the parameter P2,i hasa calculated value Bi. A specific one of the parameters P1,i or P2,i isobtained by substituting a number for the index i.

The total frequency range is indicated by FR. The subsets of the firstcalculated value(s) SUS1,i, each comprise a single calculated valueA1,i. The subsets of the second calculated value(s) SUS2,i, eachcomprise more than one (4 in the example shown in FIG. 2) calculatedvalues A2,i.

Consequently, in the associated subsets SUS1,i and SUS2,i, whichcorrespond to the same frequency sub-range SFRAi, always four secondcalculated value(s) Bi, correspond to one first calculated value(s) Ai.Each one of the four second calculated value(s) Bi, is codeddifferentially with respect to the same one first calculated value(s)Ai. This means that each of the four coded values is equal to thecorresponding second calculated value(s) Bi minus the first calculatedvalue(s) Ai.

FIG. 3 shows another schematic representation of a situation wherein thenumber of parameters during a first frame is less than during a secondframe. In contrast to FIG. 2 now the frequency sub-range obtained bycombining the frequency sub-ranges SFRB1 to SFRB4 together is notidentical to the frequency range SFRA1 but slightly smaller. Thefrequency sub-range SFRB5 occurs partly within the frequency range SFRA1and partly within the frequency range SFRA2. The coded values of theparameters P2,1 to P2,4 are coded differentially with respect to thevalue A1 of the parameter P1,1. The coded value of the parameter P2,5may be coded differentially with respect to either the value A1 or thevalue A2 of the parameter P1,2. It is also possible to code the value ofthe parameter P2,5 as the difference of the value B5 and a weighted sumof the values A1 and A2. Preferably, the values A1 and A2 are weightedin accordance with the overlap of the frequency range SFRB5 with thefrequency ranges SFRA1 and SFRA2, respectively.

FIG. 4 shows a schematic representation of a situation wherein thenumber of parameters during a first frame is higher than during a secondframe. FIG. 4 shows a similar situation as shown in FIG. 2 but now theframe t1 has a larger number of parameters P1,i than the succeedingframe t2.

The parameters P2,1 and P2,2 (further referred to as P2,i) and theirassociated frequency sub-ranges SFRB1 and SFRB2 (further referred to asSFRBi) are shown at the right side for the second frame t2. Theparameters P1,1 to P1,7 (further referred to as P1,i) and theirassociated frequency sub-ranges SFRA1 to SFRA7 (further referred to asSFRAi) are shown the at the left side for the first frame t1.

The parameter P1,i has a calculated value Ai, and the parameter P2,i hasa calculated value Bi. A specific one of the parameters P1,i or P2,i isobtained by substituting a number for the index i.

The subsets of the second calculated value(s) SUS2,i, each comprise asingle calculated value Bi. The subsets of the first calculated value(s)SUS1,i, each comprise more than one (3 in the example shown in FIG. 4)calculated values Ai.

Consequently, in the associated subsets SUS1,i and SUS2,i, whichcorrespond to the same frequency sub-range SFRBi, always one secondcalculated value(s) Bi corresponds to three first calculated value(s)Ai.

The second calculated value Bi is differentially coded with respect to acalculated weighted mean of the group of associated calculated valuesAi. The values Ai are associated with the value Bi if they belong toparameters P1,i which belong to a frequency sub-range SFRAi which occurswithin or at least partly overlaps with the frequency range SFRBi.The weighted mean is calculated as:$V_{gropup} = {\sum\limits_{i = 1}^{M}{q_{i}V_{i}}}$wherein Vgroup represents a group parameter value, M is the number ofparameters belonging to the group of associated calculated values Ai,and qi are the weight functions for which the following holds:${\sum\limits_{i = 1}^{M}q_{i}} = 1.$For example, the weights qi are selected to be 1/M, but also the size ofthe frequency sub-range or bin that a certain parameter belongs to is agood choice.

FIG. 5 shows another schematic representation of a situation wherein thenumber of parameters during a first frame is higher than during a secondframe.

In the example of FIG. 4, the bins belonging to a group in frame t1always fully fall within a single bin of frame t2. This is not the casein FIG. 5, the bin associated with the value A3 is only partly withinthe bin associated with the value B1. In differentially coding the valueB1 with respect to the weighted value, the weights for the value A3 maybe selected smaller. Preferably, the decrease of this weight is relatedto the part of the bin of A3 which is within the bin of B1 as apercentage of the bins of A1 and A2 which are completely within the binB1.

For example, the differential coding as shown in FIGS. 2 to 5 isrelevant in the parametric coding scheme as presented in E. G. PSchuijers, et.al, “Advances in Parametric coding for high-qualityaudio”, presented at 1st IEEE Benelux Workshop on Model based Processingand Coding of Audio (MPCA 2002), Leuven Belgium, Nov. 15, 2002, wherein,because of the quality/bit-rate trade-off, the number of bins used forthe IID/ITD/ICC parameters may switch to 10 or 40 frequency bins insteadof the typical 20.

FIG. 6 shows a schematic representation of a situation wherein thenumber of parameters during a first frame is less than during a secondframe.

FIGS. 2 to 5 showed a variable number of (sets of) parameters P1,i andP2,i which correspond to a certain fixed frequency region SF.Consequently, if the number of parameters changes, the size of frequencysub-ranges SFRAi or SFRBi will change accordingly such that all thefrequency sub-ranges SFRAi or SFRBi together cover the fixed frequencyregion SF.

Alternatively, as shown in FIGS. 6 and 7, each parameter P1,i and P2,imay belong to a certain frequency region SFRAi and SFRBi, respectively,i.e. the frequency region SFRAi or SFRBi a specific parameter P1,i orP2,i applies to is constant. If the number of parameters P1,i and P2,iin a frame t1 or t2 changes, the total size of the frequency rangecovered by all frequency regions SFRAi or SFRBi together changes. Thismay be the case for the ITD parameter.

In the frame t1, the left most column indicates the global parameter(s)GB1 which represent aspects of the audio signal 1 for the totalfrequency range FR. The adjacent column shows five parameters (or setsof parameters, for example IID and/or ICC parameters) which areindicated by C1 to C5. Each one of the parameters (or parameter sets) Ciis relevant for an associated frequency sub-range of the total frequencyrange FR. The frequency sub-ranges together cover the total frequencyrange FR. The right most column in the frame t1 shows two frequencysub-ranges SFRA1 and SFRA2 in which two parameters (or sets ofparameters) are defined by the values A1 and A2, respectively.

In the frame t2, the left most column indicates the global parameter(s)GB2, which correspond to the global parameter(s) GB1. The middle columnindicates the five parameters D1 to D5 which correspond to theparameters C1 to C5. The frequency ranges associated with GB1 and D1 toD5 are the same as the frequency ranges associated with GB2 and C1 toC5, respectively. The right most column in the frame t2 shows threefrequency sub-ranges SFRB1 to SFRB3 and the values B1 to B3 of theassociated parameters. The frequency sub-ranges SFRB1 and SFRB2associated with the values B1 and B2 are identical to the frequencysub-ranges SFRA1 and SFRA2 associated with the values A1 and A2,respectively. The values B1 and B2 are differentially coded with respectto the values A1 and A2, respectively. As, in the frame t1, there is nofrequency sub-range corresponding to the frequency sub-range SFRB3 inthe frame t2, it is not possible to differentially code the value B3with respect to a value in the frame t1. Still, a data reduction ispossible by coding the value B3 with respect to the global parameter(s)GB2.

Thus, in general, if the number of bins of the parameters with values Aiin a particular frame is smaller than the number of bins of thecorresponding parameters with values Bi in the next frame, thedifferential coding is performed only on bins that actually exist inboth frames. Bins that do not have a predecessor are differentiallycoded with respect to the global values GB2.

FIG. 7 shows a schematic representation of a situation wherein thenumber of parameters during a first frame is higher than during a secondframe.

In the frame t1, the left most column indicates the global parameter(s)GB1 which represent aspects of the audio signal 1 for the totalfrequency range FR. The adjacent middle column shows five parameters (orsets of parameters, for example IID and/or ICC parameters) which areindicated by C1 to C5. Each one of the parameters (or parameter sets) Ciis relevant for an associated frequency sub-range of the total frequencyrange FR. The frequency sub-ranges together cover the total frequencyrange FR. The right most column in the frame t1 shows three frequencysub-ranges SFRA1 to SFRA3 in which three parameters (or sets ofparameters) are defined by the values A1 to A3, respectively.

In the frame t2, the left most column indicates the global parameter(s)GB2, which correspond to the global parameter(s) GB1. The middle columnindicates the five parameters D1 to D5 which correspond to theparameters C1 to C5. The frequency ranges associated with GB1 and D1 toD5 are the same as the frequency ranges associated with GB2 and C1 toC5, respectively. The right most column in the frame t2 shows twofrequency sub-ranges SFRB1 and SFRB2 and the values B1 and B2 of theassociated parameters. The frequency sub-ranges SFRB1 and SFRB2associated with the values B1 and B2 are identical to the frequencysub-ranges SFRA1 and SFRA2 associated with the values A1 and A2. Thevalues B1 and B2 are differentially coded with respect to the values A1and A2, respectively.

Thus, in general, if the number of bins of the parameters with values Aiin a particular frame is larger than the number of bins of thecorresponding parameters with values Bi in the next frame, thedifferential coding is performed only on bins that actually exist inboth frames.

The coding algorithm described with respect to both FIG. 6 and FIG. 7does not require a signaling in the bit-stream.

For example, in the situation as depicted in FIGS. 6 and 7, the Ai andBi values may represent the number of ITD bins, in a practicalrealization the number of ITD bins may vary between 11 to 16.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims.

For example, the absolute number and the change thereof of parameters incorresponding bins of successive frames are examples only. In apractical situation, the number of bins may depend on the actual audiosignal and the quality of the audio to be decoded (or the availablemaximal bit stream). For example, in the situation as depicted in FIGS.6 and 7, the Ai and Bi values may represent the number of ITD bins, in aparticular practical realization the number of ITD bins may vary between11 to 16.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word “comprising” does notexclude the presence of elements or steps other than those listed in aclaim. The invention can be implemented by means of hardware comprisingseveral distinct elements, and by means of a suitably programmedcomputer. In the device claim enumerating several means, several ofthese means can be embodied by one and the same item of hardware. Themere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

1. A method of coding an audio signal, the method comprising calculatingvalues of a first number of first parameters representing aspects of theaudio signal at a first instant to obtain first calculated values,calculating values of a second number of second parameters representingthe aspects of the audio signal at a second, later, instant to obtainsecond calculated values, wherein the first number and the second numberdiffer, coding a subset of the second parameters being associated with aparticular portion of a frequency range of the audio signal based on adifference of a subset of the second calculated value(s) associated withthis particular portion of the frequency range and a subset of the firstcalculated value(s) associated with substantially this particularportion of the frequency range to obtain differentially coded values ofthe second parameters.
 2. A method of coding an audio signal as claimedin claim 1, wherein both the first parameters together and the secondparameters together cover substantially the same frequency range, andwherein the number of first parameters is smaller than the number ofsecond parameters, the subset of the first calculated value(s) comprisesone value for the particular portion of the frequency range being asub-range of the substantially the same frequency range, the subset ofthe second calculated values comprises at least two second calculatedvalues, to each one of the second calculated values corresponds one ofthe differentially coded values being based on the difference of thecorresponding second calculated value and the one value.
 3. A method ofcoding an audio signal as claimed in claim 1, wherein the firstparameters together and the second parameters together coversubstantially the same frequency range, and wherein the number of firstparameters is larger than the number of second parameters, the subset ofthe second calculated value(s) comprises one value for the particularportion of the frequency range being a sub-range of the substantiallythe same frequency range, the subset of the first parameters comprisesat least two first calculated values, the differentially coded valuecorresponding to the one value being based on the difference of a meanvalue of the corresponding first calculated values and the one value. 4.A method of coding an audio signal as claimed in claim 3, wherein themean value is calculated as a weighted sum of the first calculatedvalues with weights qi.
 5. A method of coding an audio signal as claimedin claim 4, wherein the weights qi are equal to 1\M, wherein M is thenumber of first parameters which are associated with a frequencysub-range which at least partly overlaps with the particular portion ofthe frequency range.
 6. A method of coding an audio signal as claimed inclaim 4, wherein the weights qi are related to sizes of frequencysub-ranges associated to the corresponding one of the first parameters.7. A method of coding an audio signal as claimed in claim 4, wherein theweight qi of a first parameter which is associated with a frequencysub-range which does not completely overlap with the particular portionof the frequency range of the second parameter is decreased.
 8. A methodof coding an audio signal as claimed in claim 1, the method furthercomprises calculating global values for a total frequency range of theaudio signal, and wherein each one of the first parameters and thecorresponding one of the second parameters cover substantially the samefrequency range, wherein the number of the first parameters is smallerthan the number of the second parameters, the subset of the firstcalculated value(s) comprises a value for each one of the firstparameters, the subset of the second calculated values comprises a valuefor each one of the second parameters, wherein in frequency ranges forwhich both a first and a second calculated value is calculated, thedifferentially coded value is based on the difference of thecorresponding first and second calculated value, and wherein, infrequency ranges for which a second parameter but no first parameter iscalculated, the coded value is based on the difference of thecorresponding second parameter and the global values.
 9. A method ofcoding an audio signal as claimed in claim 1, wherein each one of thefirst parameters and the corresponding one of the second parameterscover substantially the same frequency range, wherein the number offirst parameters is larger than the number of second parameters, thesubset of the first calculated value(s) comprises a value for each oneof the first parameters, the subset of the second calculated valuescomprises a value for each one of the second parameters, wherein infrequency ranges for which both a first and a second calculated value iscalculated, the differentially coded value is based on the difference ofthe corresponding first and second calculated value, and wherein infrequency ranges for which a first parameter but no second parameter iscalculated no coded values have to be determined.
 10. An encoder forcoding an audio signal and comprising means for calculating values offirst parameters representing aspects of the audio signal at a firstinstant to obtain first calculated values, means for calculating valuesof second parameters representing the aspects of the audio signal at asecond, later, instant to obtain second calculated values, wherein anumber of the first parameters and a number of the second parametersdiffer, means for coding a subset of the second parameters beingassociated with a particular portion of a frequency range of the audiosignal based on a difference of a subset of the second calculatedvalue(s) associated with this particular portion of the frequency rangeand a subset of the first calculated value(s) associated withsubstantially this particular portion of the frequency range to obtaindifferentially coded values of the second parameters.
 11. An apparatusfor supplying an audio signal, the apparatus comprising an input forreceiving an audio signal, an encoder as claimed in claim 10 forencoding the audio signal to obtain an encoded audio signal, and anoutput for supplying the encoded audio signal.